Many medical devices require electrical power to operate. Non-limiting examples of such medical devices may include pacemakers, defibrillators, drug infusion pumps, neural stimulators, ventricular assist devices (VAD), and total artificial hearts (TAH). Some devices, such as pacemakers and drug infusion pumps, require such little power that an implanted non-rechargeable battery can last for several years, reducing the need for an implantable rechargeable power source. Other devices, such as some neural stimulators, may require power levels that an implanted non-rechargeable battery cannot supply for more than a few days or weeks. These devices require the use of an implantable rechargeable battery and necessitate recharging every few days or weeks. Other relatively high-power consumption implantable devices, such as VADs and TAHs, may require power levels that an implantable rechargeable battery cannot supply for more than a few hours. With these devices, it may not be feasible to implant larger rechargeable batteries due to the size and space required. These devices necessitate recharging many times per day or the use of an external rechargeable battery pack.
A common issue encountered by powering or recharging high-power consumption implantable devices, such as VADs or TAHs, is the need for a percutaneous wire that exits the skin to transmit power from an external power source to an implanted battery or directly to the implanted device. This percutaneous wire can be a source of infection, restricts the patient from normal bathing or swimming, and can potentially leave the implanted device without power if it mechanically breaks. Some wireless power transfer systems have been developed that use inductive coupling between an implanted coil and an external coil to transfer power across the skin, thereby obviating the need for a percutaneous wire. This type of wireless power transfer system simply uses the inductive effect between two coils similar to a standard transformer. This approach has been used widely to recharge implanted batteries in some neural stimulators. However, these systems may require precise alignment between the two coils, require close spacing between coils on the order of a few inches or less, can generate significant amounts of heat near the skin, and require the patient to be immobile during charging if the external power source is not easily mobile.